Have you guys ever noticed the void?

On Wed, 11 Jan 2006 14:23:18 GMT, Richard Lamb
wrote:

No sweat, just a basic misunderstanding.

We design to a 4 G _yiield_ limit.

This is where the wing, after being stressed, no longer
returns to the original shape
It has reached the "plastic" limit and has deformed.
Yes, it has failed, but it did not break.

The 1.5 G safety factor then gives a 6 G _ultimate_ limit.
THIS is where the wing breaks.

mo better?

Richard


your brow not mine :-)

muchos mo betta.

Stealth Pilot

Having deliberated on this for a while, I'd like to try again.


For a 4 G wing (yield limit), if you pull over 4 G's, the structure
has_been_damaged - whether it came apart or not.
That's a period.


The 1.5 G safety factor _should_ give a 6 G tolerant structure, but
as has been discussed elsewhere, that should should be considered a
"should" when working on the back of an envelope.
Our limit is 4.


Operating at a higher weight, one would reduce the G limit allowed to
stay within the design envelope.


And conversely, operating at a lighter weight, one might allow a higher
G reading on the meter without exceeding the design limits.


It's all about Limits...


Which brings us to FAR Part 23 Load Factors.
Normal, utility and aerobatics categories.

Category Limit(n)Ult(n) Composite

Normal 3.8 5.7 7.6
Utility 4.4 6.6 8.8
Acrobatic 6.0 9.0 12


Normal category is limited to "non-aerobatic" flight
with no more than 60 degree banks.

Recall that a 60 degree banked coordinated turn will impose
a 2 G load on the plane (of the 3.8 G Limit)

Utility Category allows limited aerobatics, stalls, spins, etc.
and banks greater than 60 degrees.

Aerobatic category eliminates the above restrictions.


Evans(1) makes the point that "the Utility category is a good choice
for home builders because if the project turns out overweight
(more common than not) one can fall back on the normal category".


Because if weight didn't matter, neither would strength.


We'd just build them so strong they couldn't possible break
under any conditions - regardless of what is weighed...


Pop quiz:

1) An airliner at full gross is operating in what category?

2) Why the higher limits imposed on composite structures?


Richard

(1) Lightplane Designer's Handbook - Wm. S. Evans


In an infinite universe all things are possible,
unfortunately not all things are equally probable.

Richard Lamb wrote:



Pop quiz:

1) An airliner at full gross is operating in what category?

Normal

2) Why the higher limits imposed on composite structures?


Because of more variability in the strength characteristics of a
composite structure vs alum, structural scatter factor is higher
therefore more fudge factor must be included in design allowances.

John

Montreal